1. Field of the Invention
The present invention relates to acoustic wave elements used in resonators, bandpass filters, and the like. Particularly, the present invention relates to an acoustic wave element having an improved structure of an IDT electrode including a plurality of electrode fingers.
2. Description of the Related Art
Acoustic wave elements have been widely used in resonators and bandpass filters. As the acoustic wave elements, surface acoustic wave (SAW) elements utilizing surface acoustic waves and boundary acoustic wave (BAW) elements utilizing boundary acoustic waves are known.
In the acoustic wave elements such as the BAW elements and the SAW elements, an IDT electrode including a plurality of electrode fingers is used to excite acoustic waves. By applying crossing width weighting to the IDT electrode, filter characteristics and resonance characteristics are improved. However, even when a weighted IDT electrode is used, it has been difficult to obtain sufficient resonance characteristics and filter characteristics.
WO2006/109591 discloses an acoustic wave element having an improved shape of an IDT electrode to further improve resonance characteristics and filter characteristics.
FIG. 28 is a schematic partial plan view illustrating a main portion of an IDT electrode of the acoustic wave element described in WO2006/109591. As illustrated in FIG. 28, in an IDT electrode 1000, a plurality of electrode fingers 1001 connected to one potential and a plurality of electrode fingers 1002 connected to the other potential are alternately arranged in an acoustic wave propagation direction.
In the portion illustrated in FIG. 28, a dummy electrode finger 1004 is located at the end of the first electrode finger 1001 via a gap 1003 in an electrode finger length direction. A protrusion 1005 is formed on a side edge of the second electrode finger 1002. The protrusion 1005 is located at a position substantially equal to the position of the gap 1003 along the electrode finger length direction on one of the side edges of the second electrode finger 1002. In the portion where the gap 1003 exists, a dimension in the width direction along the acoustic wave propagation direction of a region with no electrode is small in accordance with the protrusion 1005. This structure reduces the difference in behavior between acoustic waves passing the gap 1003 and acoustic waves propagating in a portion where the gap 1003 does not exist, whereby the resonance characteristics and filter characteristics are improved.
Also, in the IDT electrode 1000, the end of the first electrode finger 1001, i.e., a side end portion of the gap 1003, and the end of the dummy electrode finger 1004, i.e., the end portion on the gap 1003 side, have tapered portions 1001a and 1004a, respectively, which taper down toward the ends. Providing the tapered portions 1001a and 1004a causes a gradual change in propagation environment of acoustic waves from the side of the gap 1003 toward the opposite side in the portions provided with the tapered portions 1001a and 1004a. This change also improves the resonance characteristics and filter characteristics.
As described above, in the acoustic wave element including the IDT electrode 1000 described in WO2006/109591, the resonance characteristics and filter characteristics are improved to some extent due to the devised shape of the IDT electrode compared to a conventional acoustic wave element. However, the improvement is insufficient. That is, a further improvement of the resonance characteristics and filter characteristics is intensively demanded.
In the structure described in WO2006/109591, when the protrusion 1005 is larger and when the tapered portions 1001a and 1004a are larger, the portion touches an adjoining electrode finger connected to the other potential, which causes problems of short circuit and degradation in insulation properties.
Furthermore, the characteristic at the vicinity of a resonance point heavily depends on the shape of the IDT electrode. Therefore, the above-described shape disadvantageously causes variations in the shape of the electrode fingers during manufacturing, leading to variations in resonance characteristics and filter characteristics. In addition, when the size of the protrusion 1005 is increased to enhance the resonance characteristics and filter characteristics, a stopband becomes narrow disadvantageously.